Cellulose fiber-based compositions and their method of manufacture

ABSTRACT

An improved method for refining cellulose that produces a highly refined cellulosic material comprising soaking raw material in a mild NaOH using reduced temperatures and pressures, and refining the material with a plate refiner so that a waste water stream is reduced in volume and has a pH within a range of 8 to 9. Also, a HRC gel having a lignin concentration of at least about one (1)% by weight, and a water retention capacity of about 25 to at least about 56 g H 2 O/g dry HRC is provided.

FIELD

The present invention relates to the manufacture of compositions fromcellulose fiber material and the products obtained.

BACKGROUND

Many food and agricultural byproducts contain substantial amounts ofcellulose. Cellulose is known to be useful in a wide range of markets.The food industry uses cellulose as a fat replacer, a component inproducts such as dietary fiber supplements, suspension agents,emulsifiers, water binding agents, as well as for edible films andcoatings. The pharmaceutical industry uses cellulose as a component inbulking agents and dietary fibers for treatment and prevention ofcoronary heart disease, Type II diabetes, hypertension, diverticulardisease, hemorrhoids, obesity, and so forth. Industrial applications ofcellulose include use in filter medias, latex paint, and so forth.

Native cellulose fibers contain lignin, a polymeric material found inevery type of vascular plant. Prior art processes for refining celluloseseek to remove lignin before any substantive treatment of the fibers.Lignin is known to cause cellulose fibers to stick together, thusreducing the surface area available for any subsequent reactions. It isbelieved that the presence of lignin also reduces the ability ofcellulose microfibers to intertwine and entangle, thus reducing thestructural integrity and/or strength of the final product.

Lignin removal is currently accomplished using extremely hightemperatures and pressures. These extreme conditions cause raw materialfragments to break apart, thus releasing the desired cellulose-basedmicro fibers. In addition, the raw materials are subjected to highconcentrations of sodium hydroxide. See, for example, U.S. Pat. No.5,817,381 to Chen, et al. Such a process is extremely energy-intensivein terms of the required temperatures and pressures. Further, theprocess produces a waste stream regarded as hazardous due to elevated pHlevels caused by the use of large amounts of sodium hydroxide. Treatmentof the waste stream adds to the cost of production and impacts theoverall efficiency of this process.

Therefore, what is needed is a more efficient method of refiningcellulose which is also safe for the environment.

SUMMARY

The present invention comprises a method for refining cellulosecomprising soaking raw material in NaOH having a concentration of aboutfive (5) to 50% (dry basis) to produce soaked raw material which steepsfor about 6 hours to allow the NaOH to work, refining the soaked rawmaterial to produce refined material, dispersing the refined material toproduce dispersed refined material, and homogenizing the dispersedrefined material to produce highly refined cellulose (HRC) gel having alignin concentration of at least about one (1)% and a water retentioncapacity (WRC) of about 25 to at least about 56 g H₂O/g dry HRC. Themethod of the present invention produces a waste stream having a pHwithin a range of 8 to 9 and a reduced volume as compared toconventional cellulose refining processes.

In one embodiment, the method further comprises draining and washing thesoaked raw material until the pH is down to about 8 to 9, bleaching thewashed material at a temperature of about 20 to 100° C. in hydrogenperoxide having a concentration of about one (1) to 20% dry basis, andwashing and filtering the bleached material to produce a filteredmaterial having a solids content of about thirty (30)%.

In one embodiment, the filtered material is refined by being passedthrough a plate refiner. The plate refiner essentially breaks up thelignin as it shreds the material into refined cellulose particles.

In another embodiment, a cellulosic material having a ligninconcentration of about 10 to 20% is described having a WRC of about 25to at least about 56 g H₂O/g dry HRC. In another embodiment a HRC powderis described having a WRC of about 20 to 40 g H₂O/g dry HRC. Thesefunctional characteristics are known to be related to average porediameter and surface area of the HRC material.

The method of the present invention is energy efficient because it doesnot require high pressures and temperatures as in prior art processes.Despite the presence of higher lignin concentrations in the finalproduct, the HRC gel of the present invention has a water holdingcapacity that is at least as good or better than prior art products. Useof a plate refiner to break up the lignin rather than using highconcentrations of NaOH has the added advantage of producing anon-hazardous waste stream having pH within a range of 8 to 9 and areduced volume.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram illustrating a process for producinghighly refined cellulose material in one embodiment of the presentinvention.

FIG. 2 is a graph showing water retention capacity (WRC) in grams ofwater per grams of dry highly-refined cellulose (HRC) versus NaOHconcentration in grams of NaOH per grams of water as described in theExample herein.

FIG. 3 is a graph showing ORC in grams per 100 grams of dry HRC versusNaOH concentration in grams of NaOH per grams of water as described inthe Example herein.

FIG. 4 is a graph showing surface area in square meters per gram andaverage pore diameter in Angstroms (Å) versus NaOH concentration ingrams of NaOH per grams of water as described in the Example herein.

DETAILED DESCRIPTION

A new process for making HRC cellulose from agricultural byproducts suchas corn cobs, husks, and stalks is disclosed. The HRC dispersion of thepresent invention is a highly viscous, semi-translucent gel. HRCembodiments comprise dried powders that are redispersable in water toform gel-like solutions. The functional characteristics of HRC arerelated to various properties, including water- and oil-retentioncapacity, average pore size, and surface area.

The method of the present invention for making HRC includes soakingchopped raw material, such as corn cobs, husks and stalks, in a mildNaOH solution. The soaked material is then drained and washed with tapwater. This is followed by a bleaching step with hydrogen peroxide. Thebleached material is washed and filtered before being subjected to aplate refiner which shreds the material into micro fibers. The microfibers are passed through a screen filter and diluted to about one (1)%solids and dispersed before being homogenized at high pressure toproduce HRC gel.

The present invention also includes an aqueous HRC gel having a ligninconcentration of about one to twenty (1 to 20)%. The present inventionalso includes a freeze-dried HRC that has a lignin concentration of notmore than about one to twenty (1 to 20)% . The HRC products of thepresent invention exhibit a surprisingly high WRC in the range of about20 to at least about 56 g H₂O/g dry HRC. This high WRC is at least asgood as, and in some cases, better than the WRC of prior art productshaving lower lignin concentrations. The HRC products exhibit similargood properties for ORC.

As shown in FIG. 1, the process of the present invention begins when rawmaterial 110 is chopped 112 with a chopper into pieces less than orequal to about 5 cm×2 cm. Any conventional type of manual or automatedchopper can be used, such as a knife or a larger commercially-sizedchopper. The resulting chopped raw material 114 is then washed anddrained 116, thus removing dirt and unwanted foreign materials. Thewashed and chopped raw material 118 is then soaked 120 in a NaOHsolution having any suitable concentration, such as about one (1)%. Inone embodiment, the NaOH concentration is about five (5) to 50% (drybasis). In another embodiment, the NaOH concentration is about 0.00357to 0.025 g NaOH/g H₂O. Although prior art processes also soak cellulosein NaOH, the concentrations used are much higher, such as about ninety(90)% by weight or higher. The bath of NaOH is kept at a temperature ofabout 20 to 100° C. The temperature of the NaOH is maintained withinthis range in order to soften the material. The actual soaking timedepends on NaOH concentration and temperature. In one embodiment, about100 g of chopped raw material 114 is soaked in a 2.5 liter bath within atemperature range of about 20 to 80 degrees Centigrade for 10 to 90minutes.

After having the chopped raw material sit for about 6 hours, theresulting soaked raw material 122 is subjected to another washing anddraining 124 until the pH is down to around eight (8) to nine (9). Inone embodiment, the soaked raw material 122 is washed with tap water. Inone embodiment, the material is drained. This is followed by bleaching126 the material with hydrogen peroxide at concentrations of about one(1) to 20% (dry basis). The material is bleached at about 20 to 100° C.for about five (5) to 200 min. The bleached material 128 is thensubjected to washing with water, followed by filtering 130 with ascreen. The screen can be any suitable size. In one embodiment, thescreen has a mesh size of about 30 to 200 microns.

The filtered material 132 containing solids is then refined 134 in aplate refiner. In one embodiment, the filtered material 132 entering theplate refiner contains about four (4)% solids. The plate refinereffectively shreds the particles to create microfibers 136. The platerefiner, which is also called a disk mill, comprises a main body withtwo ridged steel plates for grinding materials. One plate, a refiningplate, is rotated while a second plate remains stationary. The platesdefine grooves that aid in grinding. One plate refiner is manufacturedby Sprout Waldron of Muncy, Pa. and is Model 105A. This plate refinerhas a 40 horsepower motor that operates at 1775 rpm.

Water is fed into the refiner to assist in keeping the solids flowingwithout plugging. Water prevents the refiner's plates from overheating,which causes materials in the refiner to burn. The distance between theplates is adjustable on the refiner. To set refining plate distances, anumbered dial was affixed to the refining plate adjustment handle. Thedistance between the plates was measured with a micrometer, and thecorresponding number on the dial was recorded. Several plate distanceswere evaluated and the setting number was recorded. A variety of flowconsistencies were used in the refiner, which was adjusted by varyingsolids feed rate. The amount of water flowing through the refinerremained constant. Samples were sent through the refiner multiple times.In one embodiment the materials are passed one or more times through theplate refiner.

The microfibers 136 are then separated 138 with a centrifuge to producerefined materials 140. The refined materials 140 are then diluted 142 inwater until the solids content is about 0.5 to 37%. This material isthen dispersed 144. In one embodiment, dispersement continues until asubstantially uniform suspension is obtained, about 2 to 10 minutes. Theuniform suspension reduces the likelihood of plugging.

The resulting dispersed refined materials, i.e., microparticles 146, arehomogenized 148 in any known homogenizer operating at a suitablepressure. In one embodiment, pressures greater than about 5,000 psi areused. The resulting highly refined cellulose (HRC) gel 150 has a lignincontent of about 1 to 20% by weight.

The use of a mild NaOH soaking and a refining step in the presentinvention prior to high pressure homogenization avoids the requirementof high temperature and high pressure cooking. This novel processfurther avoids the use of highly concentrated NaOH and the associatedundesirable environmental impact of discharging waste water containinghigh levels of NaOH and organic compounds. The process also avoids aneed for an extensive recovery system. In one embodiment, the pH of thedischarge stream in the present invention is only about 8 to 9. Themethod of the present invention has the further advantage of reducingwater usage significantly over prior art processes, using only aboutone-half the amount of water as is used in conventional processes toproduce to produce HRC gel.

In another embodiment, the HRC products of the present invention possessa WRC and ORC that are at least as good or even better than prior artproducts. This is true even though the products of the present inventionhave a higher lignin concentration than products made using conventionalprocesses. It is assumed that the lignin which is present has beensubstantially inactivated to a sufficient degree so that the undesirableclumping did not subsequently occur. Another reason for these improvedproperties may be due to a porous network structure that is present inthe HRC products of the present invention, but is lost in prior artproducts due to high concentration soaking in NaOH.

The invention will be further described by reference to the followingexample which is offered in order to further illustrate the presentinvention and the advantages thereof. This example is in no way meant tobe limiting, but merely illustrative.

EXAMPLE HRC Sample Preparation

The HRC samples were prepared according to the process described abovein FIG. 1. In this experiment, dry corn stalks were used as the rawmaterial. Specifically, about 100 g of dry corn stalks were chopped intosmall pieces. The chopped pieces were then soaked in NaOH solutions withconcentrations from about 0.00357 to 0.025 g NaOH/g H₂O for at leastabout 1 hour. After soaking, the material was drained and was allowed tosit for about 6 hours. The soaked material was then washed with tapwater and bleached with hydrogen peroxide. Refining of the bleachedmaterial was performed with a commercial refiner. The refined materialwas then diluted to about 1% solids and was dispersed for about 5 to 10minutes. High-pressure homogenization of the dispersed material wasaccomplished using a conventional homogenizer.

A portion of the resulting HRC gel was freeze-dried. Dried HRC sampleswere prepared using a freeze dryer, model RVT 4104-120 from SavantInstrument Inc. The samples were dried at −180° C. and zero (0) mmHgvacuum.

Determination of Water-Retention Capacity (WRC) and Oil-RetentionCapacity (ORC)

WRC is a measure of the amount of water retained under standardcentrifuge. The WRC values for both aqueous HRC gel and freeze-dried HRCwere determined in accordance with Method 56-10 of the AmericanAssociation of Cereal Chemists (AACC).

Determination of Pore Size and Microsurface Area

Both pore size and microsurface area of freeze-dried HRC samples weremeasured using a Micromeritics 2000 from Micromeritice Instrument Co.The test sample was weighed with a precision of 0.0001 g. In all cases,the test sample weight was more than 100 mg to reduce the effect ofweighing errors. At 85° C. and 6 mmHg vacuum, the sample was degassed,and moisture and other contaminants were removed. The degassed samplewas analyzed in a nitrogen gas environment. Average pore diameter, BETsurface area and Langmuir surface area were measured. The BET surfacearea values were determined by calculating the monolayer volume ofadsorbed gas from the isotherm data. The Langmuir surface area valueswere obtained by relating the surface area to the volume of gas adsorbedas a monolayer.

Results and Discussion

WRC and ORC

WRC values were measured for both the aqueous HRC gel and dried HRCpowder using NaOH concentrations ranging from about 0.004 to 0.025 gNaOH/g water as shown in FIG. 2. The WRC values for both the HRC gel andHRC powder were in the range of about 20 to at least about 56 g H₂O/gdry HRC, depending on the concentration of the alkaline solutions.Maximum WRC values for the gel of at least about 56 g H₂O/g dry HRC wereobtained with a NaOH concentration of about 0.007 g NaOH/g H₂O. Dryingthe HRC gel resulted in a reduction of about three (3) to 15% in WRC,which may be attributed to structural damages such as recrystallizationcaused by dehydration. However, the HRC powder also exhibited high WRCvalues, having a maximum WRC value of at least about 56 g H₂O/g dry HRCat a NaOH concentration of about 0.007 g NaOH/g H₂O.

Compared with WRC values of 3.5 to 10 g water/g dry powdered cellulosereported by Ang and Miller in Cereal Foods World, Multiple Functions ofPowdered Cellulose as a Food Ingredient, Vol. 36 (7): 558-564 (1991),both the HRC gel and powder of the present invention have a much higherwater-holding capacity.

Similar behavior was observed for ORC as shown in FIG. 3. ORC values forHRC gel ranged from about 10 to 19 g/100 g dry HRC in NaOHconcentrations ranging from about one (1) to four (4) g NaOH/g water.HRC gel also showed a maximum ORC of about 19 g NaOH/g water at a NaOHconcentration of about two (2) g NaOH/g water. These results aresurprising in light of the lignin concentrations of about 1 to 20% byweight.

One possible reason for the improved WRC and ORC characteristicsexhibited by the HRC gel and HRC powder of the present invention is thathigh concentration soaking might result in a microstructure unfavorablefor holding water molecules.

Pore Size and Surface Area

Average pore size is a measure of openness of the HRC structure. FIG. 4shows that the average pore size increased rapidly as NaOH concentrationwas increased to 0.007%, then slowly with further increase in NaOHconcentration. The surface area reached a maximum value at 0.007% NaOH,which also coincides with the maximum WRC discussed above. The decreasein surface area after the maximum value seems to suggest an increase inthe ratio of large pores to small pores, which may contribute to thedecrease in total surface area.

In one embodiment, the process of the present invention removes ligninto a sufficient degree or substantially inactivates it such thatundesirable fiber clumping does not occur.

The HRC products of the present invention have uses in substrates wherewater binding or oil binding is important such as in foods, soil, and infiltration media. The HRC products also have use in applications where alarge surface area is important, such as accelerating chemicalreactions.

It is to be appreciated that the method and product of the presentinvention have been described in particular detail with respect topreferred processes and formulations. The present invention, however, isnot intended to be limited to these preferred embodiments. One skilledin the art will readily recognize that the actual method and product maybe adjusted to accommodate particular conditions.

What is claimed is:
 1. A method for refining cellulose, comprising:providing raw material comprising cellulose and chopping the rawmaterial; soaking chopped raw material in NaOH and steeping the materialat least about 6 hours to produce steeped raw material; washing anddraining the raw material until the pH of wash water is 8 to 9; refiningthe steeped raw material comprising about 4% solid particles to shredand produce refined material comprising microfibers; separating themicrofibers from the refined material and diluting the microfibers withwater to form a suspension; dispersing the refined material to producedispersed refined material; and homogenizing the dispersed refinedmaterial at a pressure greater than about 5000 psi, to produce highlyrefined cellulose (HRC) gel having a lignin concentration of at leastone (1)% to about twenty (20)% and a water retention capacity (WRC) ofabout 25 to at least about 56 g H₂O/g dry HRC.
 2. The method of claim 1further comprising drying the HRC to produce HRC powder having a WRC ofabout 20 to at least about 40 g H₂O/g dry HRC.
 3. The method of claim 2wherein the NaOH concentration is about 20% and the WRC is at leastabout 40 g H₂O/g dry HRC.
 4. The method of claim 1 wherein the rawmaterial is steeped in NaOH at a temperature ranging from about 20 to100° C.
 5. The method of claim 1 wherein the NaOH concentration is about1% and the WRC is at least about 56 g H₂O/g dry HRC.
 6. The method ofclaim 1 further comprising chopping and washing the raw material priorto steeping.
 7. The method of claim 1 further comprising draining andwashing the steeped raw material in wash water to produce washedmaterial; bleaching the washed material in hydrogen peroxide to producea bleached material; and washing and filtering the bleached material toproduce a filtered material wherein the wash water is of a volumeeffective for reducing pH within a range of 8 to
 9. 8. The method ofclaim 7 wherein the steeped raw material is washed until the pH of thewash water is within a range of 8 to
 9. 9. The method of claim 7 whereinthe hydrogen peroxide has a concentration of about one (1) to twenty(20)% dry basis, further wherein the washed material is bleached at atemperature of about 20 to 100° C.
 10. The method of claim 7 wherein thefiltered material has a solids content of about 4%.
 11. The method ofclaim 10 wherein the filtered material is refined in a plate refiner.12. The method of claim 11 wherein the filtered material passes throughthe plate refiner more than once.
 13. A product made by the method ofclaim
 1. 14. A method for refining amorphous cellulose, comprising:soaking raw material in NaOH and steeping the material for at leastabout 6 hours to produce steeped raw materials; and refining the steepedraw material comprising about 4% solids particles to shred and producerefined material comprising microfibers wherein the method produces awaste water stream having a pH within a range of 8 to 9.